Pumps in the form of feed screws are known for their conveyance of fluid continuously without formation of foam. These screw pumps contain a drive screw supported in the intake and discharge chambers and one or several secondary screws.
Such pumps or feed devices comprise meshing or opposing screws having constantly converging threads supported in an "S"-shaped housing. One screw is power driven. The secondary screw or screws are usually driven by the thread flanks or edges of the main screw at a higher driving force or ratio. They may also be driven by a gear connection to the shaft of the main screw. The axial forces resulting from the pressure build up of the conveyed fluid are usually taken up by a roller or axial sleeve bearing supported on the drive screw.
Screw pumps or feeds of this design have the drawback that it is not possible to adjust the clearance between the threads in accordance with the properties of the transported material, since a sufficiently large range of clearances is not available. Thus it is impossible, for instance, to transport plastic materials filled with quartz powder.